Revision notes

1. CDI Revision Notes
Term 1 (2017 – 2018)
Grade 12 Advanced
Unit 2 – Mechanical Systems
STUDENT INSTRUCTIONS –
• Student must attempt all questions.
• For this examination, you must have:
(a) An ink pen – blue.
(b) A pencil.
(c) A ruler.
(d) A calculator (if required).
• Electronic devices are not allowed.
Examination Specifications
Domain Marks Time
Section 1 - 5 Multiple Choice Questions 5 Marks 3 - 4 minutes
Section 2 - 5 True or False Statements 5 Marks 3 - 4 minutes
Section 3 - 2 Short answer Questions
2 Diagram Questions
1 Matching Task
10 Marks (2 x 5)
20 Marks (2 x 10)
10 Marks
8 - 10 minutes
10 – 12 minutes
3 – 5 minutes
Total – 50 Marks Total – 35 minutes
(5 minutes reading)

2. SECTION 1 -
MECHANICAL SYSTEMS
Word Meaning Image
Fluid A substance , either a gas or liquid, which
flows and conforms to the shape of it
container.
Hydraulic system A fluid power system, which transmits force
through an incompressible fluid.
Pneumatic
system
A fluid power system, which transmits force
through a compressible fluid.
Hydraulic seal A relatively soft, non metallic ring, captured in
a groove or fixed in a combination of rings,
formed to black or separate fluid in
reciprocating motion applications.

3. AIR COMPRESSORS
AIR BRAKES
DENTAL DRILLS
CAR LIFTS
CRANES
HEDGE CUTTERS
APPLICATIONS OF HYDRAULIC
SYSTEMS
APPLICATIONS OF PNEUMATIC
SYSTEMS

4. • All liquids have a high resistance to compression.
• The example shows two cylinders of equal volume,
each fitted with pistons.
• One containing liquid and the other one containing air.
• A force of 20,000 N (Newton’s) is applied to both
pistons.
• The decrease in volume of the air is very large
compared to that of the liquid .
COMPRESSIBILITY OF
FLUID
PASCAL’S LAW
• Pressure in an enclosed container is transmitted equally and undiminished to all
parts of the container and acts at right angles to the enclosing walls.
• Force applied at one point is transmitted to another point using fluid that can’t be
compressed .
QR CODE FOR
PRINCIPLE OF
HYDRAULIC LIFT

5. Problem:
A hydraulic system is used to lift a 2000-kg vehicle in an auto garage,
as shown in the figure given below. If the vehicle sits on a piston of
area 1m2, and a force is applied to a piston of area 0.05m2, what is
the minimum force that must be applied to lift the vehicle?
Solution:
A1 = 0.05 m2; A2 = 1 m2; W = 2000 kg; g = 9.8 m/s2
𝐹1
𝐴1
=
𝐹2
𝐴2
𝐹1 =
𝐹2
𝐴2
𝑋 𝐴1
𝐹1 =
2000 𝑋 9.8
1
𝑋 0.05
𝐹1 = 100 𝑋 9.8 = 980 𝑁

6. SECTION 2 -
BASIC HYDRAULIC
SYSTEM
Word Meaning Image
Hydraulic
Pump
A mechanical device that
converts mechanical power into
hydraulic energy, to generate
hydraulic pressure.
Actuator A component that converts a
type of energy to mechanical
energy to move a movable part
in the system.
Linear
actuator
An actuator that creates motion
in a straight line.
Reservoir A container for holding the fluid
required to supply the system,
including a reserve to cover any
losses from minor leakages and
evaporation.
Word Meaning Image
Non –
return valve
A one – way valve that normally
allows fluid (liquid or gas) to flow
through it in one direction only,
also called “check valve”.
Selector
Valve
A directional control device that
ensures the movement of the
hydraulic fluid flow in the proper
direction.
Pressure
relief valve
A safety device designed to
protect a pressurized system
during an overpressure event.
Hydraulic
filter
A device that removes
contaminants from the hydraulic
fluids.

7. Hydraulic fluid is stored
here.
1. It draws the hydraulic fluid
through a NRV before it.
2. Then it pushes the fluid
through another NRV via the
pressure pipeline.
The fluid is then pushed to the selector valve and depending on its position.
1. the fluid is directed through a port to one side of a double acting linear
actuator piston - OR
2. the fluid will be locked in an actuator and there is no movement of the piston.
Fluid from the non pressure
side of the actuator piston is
directed back to the reservoir
by another port in the selector
valve via the return pipeline.
• Non – pressurized
fluid in green
• Pressurized fluid in
red
• Return fluid in yellow
SIMPLE HYDRAULIC SYSTEM

8. CAR BRAKE SYSTEM
1. The driver presses the brake pedal.
2. A spring is actuated.
3. The piston forces the fluid through a series of brake lines to
a slave cylinder.
1. Fluid – Hydraulic brake fluid
2. Slave Cylinder – located at each wheel which applies
force to the brake pads creating friction between the
rotation drum or disc and slowing wheel rotation.
Slave cylinders are mounted
to the back – plate on
a drum brake system
A slave cylinder is incorporated
into the brake caliper
on a disc brake system

9. SECTION 3 -
ADVANCE HYDRAULIC SYSTEM
Word Meaning Image
Flap actuator It is used to move the flap
(aerofoil attached to the
back of the wing) in the
aircraft.
Low –
pressure
return filter
A filter located in the return
line, just before the fluid
enters the reservoir.
Word Meaning Image
Engine driven
pump
A pump is driven by the
engine gearbox to give the
hydraulic system a certain
amount of pressure and
flow.
Accumulator A component which stores
energy in the form of
hydraulic pressure.
Unloading
valve
A valve that relieves the
pressure off the pump by
delivering the fluid back to
the reservoir.

10. LIGHT AIRCRAFT HYDRAULIC SYSTEM
Hydraulic fluid is
stored in reservoir
either to the EDP
(engine driven pump)
or to the hand pump.
The EDP builds the
pressure in the system
and pushes the fluid to
the selector valve and
the actuator.
The fluid from the pump to the actuator passes through
unloading valve, pressure relief valve and accumulator.
The hydraulic pressure
will act on the flap
actuator to move the flap
surface in the aircraft.
The fluid will go back to the reservoir via the return line.
It removes the contamination from the system and the
fluid, before the fluid returns back to the reservoir.

11. SECTION 4 -
PNEUMATIC SYSTEMS
Word Meaning
Pneumatic Containing or operated by air or gas under
pressure.
Pneumatic
system
A fluid power system which transmits force
through compressible air.
Compresse
d air
Air that is at a higher pressure than
atmospheric pressure.
Vacuum A space entirely devoid of matter.
Valve A device for controlling the passage of liquid or
gas (air) through a pipe, duct, etc.
Innovation A uniquely new idea, device or method.
Component Function
Receiver tank It stores the air that comes to the compressor.
Compressor It increases the pressure of the air.
Air filters Used to remove the contaminants from the air.
Control Valves These are used to regulate, control and monitor pressure
and control flow direction.
Air actuator It is a component of a machine that is responsible for
moving or controlling a mechanism and operated by
pneumatic power.
Air cooler It reduces the temperature of the compressed air during
the compression process.

12. INNOVATION AND THE FUTURE OF PNEUMATICS
Pneumatic Transport - Hyperloop
How does the Hyperloop work?
1. There’s a tube (tunnel) from point A to point B.
2. The air from the tube has been removed; i.e., low air pressure is present inside the tube.
3. Then there are some pods inside that tube.
4. These pods will levitate and move forward tackling friction and reducing drag (as the air has already been removed).
5. For some air left in that tube, compressor fans installed in the front of the pods will suck in the air and push them via air bearings.
6. Anything can be put inside the pods, depending upon its dimensions and capacity.
7. These pods will move from point A to point B, moving the things (or people) in them. Imagine this. You have a tube with low air pressure in
it. Now you are shooting pods inside it at high speeds